1. Field of the Invention
The present invention relates to a circuit for eliminating noises at power on and off and, more particularly, to a circuit for eliminating pop sounds at power on and off by a moderate waveform.
2. Description of Related Art
Upon the rapid advance of electronic technology, multimedia applications are increasingly complicated. In addition to the requirement of image quality, the sound quality is required higher and higher. FIG. 1 is a schematic diagram of a typical sound signal. As shown in FIG. 1, due to the sampling, the range of the sound signal is from positive voltage to negative voltage. However, an operating voltage for a typical integrated circuit (IC) ranges from zero to VDD volts, and the sound signal is biased to VDD/2.
When the sound signal is biased, it directly adds a DC bias VDD/2 and the sound signal, which can cause a pop noise sounding “Bo” at power on due to the discontinuous DC operating voltage. Likewise, the DC operating voltage for the sound signal at power off is rapidly reduced from VDD/2 volts to zero, and the pop noise “Bo” is present. As shown in FIGS. 2A and 2B, when an output voltage of a digital to analog converter (DAC) is changed from zero to VDD/2 volts, the DAC adds the internal sound signal to the DC operating voltage. Such a way gradually increases the DC operating voltage from zero to VDD/2 volts, so the pop noise is not clearly present except for the positions B and C where the DC operating voltage is discontinuous. In addition, the DC operating voltage is increased exponentially, not linearly, when the DAC adjusts the DC operating voltage from zero to VDD/2 volts. This may cause slight noises. Further, the entire system response speed is typically limited by the DAC, and the system cannot be powered on or off rapidly. For rapid power-on or -off, it requires a DAC with the fast response speed, which relatively increases the system cost.